Macrolides are multi-membered lactone rings having one or more deoxy sugars as substituents. Erythromycin, azithromycin, and clarithromycin are macrolides that have bacteriostatic and/or bactericidal activity. Ascomycin, tacrolimus, and Pimecrolimus are also macrolides.
Ascomycin (CAS No. 11011-38-4) is an immunomodulating macrolactam that reportedly blocks T-cell activation, inhibits cytokine release, and inhibits mast cell activation. “The mechanism of action of ascomycin is very similar to that of cyclosporin and of tacrolimus, although the three compounds have different chemical structures.” C. E. Griffiths, Ascomycin: An Advance in the Management of Atopic Dermatitis; Br. J. Dermatol., 144(4), 679-681 (April 2001). Ascomycin was disclosed in U.S. Pat. No. 3,244,592, where the compound is described as an antifungal agent, and it has the chemical formula:

The use of ascomycin as an immunosuppressant is described in European Patent Application No. 323865.
The crystalline form of a solid chemical compound (or the lack of a crystalline form) affects many of the compound's properties that are important with respect to formulation as a pharmaceutical. Such properties include, for example, the flowability of the milled solid. Flowability affects the ease with which the material is handled during processing into a pharmaceutical product. When particles of the powdered compound do not flow past each other easily, a formulation specialist must take that fact into account in developing a tablet or capsule formulation, which may necessitate the use of glidants such as colloidal silicon dioxide, talc, starch or tribasic calcium phosphate.
Another important property of a pharmaceutical compound that may depend on crystallinity is its rate of dissolution in aqueous fluid. The rate of dissolution of an active ingredient in a patient's stomach fluid can have therapeutic consequences since it imposes an upper limit on the rate at which an orally-administered active ingredient can reach the patient's bloodstream. The solid state form of a compound may also affect its behavior on compaction and its storage stability.
These practical physical characteristics are influenced by the conformation and orientation of molecules in the unit cell, which defines a particular crystalline form of a substance. These conformational and orientation factors in turn result in particular intramolecular interactions such that different crystalline forms may give rise to distinct spectroscopic properties that may be detectable by such analytical techniques as powder X-ray diffraction, solid state 13C NMR spectrometry, and infrared spectrometry. A particular crystalline form may also give rise to thermal behavior different from that of the amorphous material or another crystalline form. Thermal behavior is measured in the laboratory by such techniques as capillary melting point, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and can be used to distinguish some crystalline forms from others.
The discovery of new crystalline forms of a pharmaceutically useful compound provides a new opportunity to improve the performance characteristics of a pharmaceutical product. It enlarges the repertoire of materials that a formulation scientist has available for designing, for example, a pharmaceutical dosage form of a drug with a targeted release profile or other desired characteristic. Therefore, there is a need in the art for crystalline forms of Ascomycin.